Open Access

Distinct S-wave reflectors (bright spots) detected beneath the Nagamachi-Rifu fault, NE Japan

  • Norihito Umino1Email author,
  • Hisako Ujikawa1,
  • Shuichiro Hori1 and
  • Akira Hasegawa1
Earth, Planets and Space201454:BF03353295

Received: 8 January 2002

Accepted: 26 September 2002

Published: 26 June 2014


Distinct phases reflected from mid-crustal reflectors (SxS and PxP phases) were observed in seismograms of aftershocks of 1998 M5.0 Sendai earthquake at nearby stations. We estimated the locations of the reflectors (bright spots) by using arrival time differences between these phases and direct waves. A clear reflector is located in the depth range of 15 to 21 km just beneath the fault plane of the M5.0 event. It dips toward the NNE direction with a dip angle of about 25°. Other reflectors are also located beneath the fault plane of the event. Internal structure of the S-wave reflector was estimated from spectral amplitude ratios of reflected SxS-wave to direct S-wave. Observed spectral ratios show that S-wave velocity in the reflector body is 1.1 km/s, and its thickness is about 50 m. This suggests that a thin reflector body (bright spot) partially filled with fluids exists in the lower crust beneath the focal area of the 1998 event.